Construction Of Lithium Ion Capacitor With High Specific Energy Density And Its Electrochemical Performance

Lithium ion capacitors ?LICs?, inherit the advantages of supercapacitors, such as high power density, good cycle performance and fast charge-discharge capability. The energy density of LICs is greatly improved by using electrode materials of lithium ion battery. Hence, LICs have broad application prospects in electronics, electric vehicles, energy storage and many other fields. Moreover, the energy density of LICs still needs to be improved when it is compared to lithium ion battery. The construction of novel asymmetric capacitor and the synthesis of related electrode materials have recently drawn considerable attentions in both industry and research communities. To increase the energy density of LICs, we constructed new LIC systems and investigated the correlation between microscopic structure of electrode materials and their electrochemical performance in this dissertation. The research includes two sections:increasing specific capacitance and broadening potential window, which are based on the formula of energy density E=1/2CV2. The specific content includes the following aspects:?1? For Carbon//Li₄Ti₅O₁₂ ?C//LTO? LICs, positive electrode of porous carbon limits the capacitance of the LICs. The influence of porous structure ?e.g. surface area, pore volume? on electrochemical performance in three-electrode system and two-electrode system of LICs was studied. The results show that the pore volume is a key factor of the capacitance in two-electrode system. The pore volume of three mesoporous carbons is in range of2.4?4.3cm3 g-1 and the capacitances of LICs based on mesoporous carbons are in range of 26?39Fg-1, which are higher than those of microporous carbon and micro-mesoporous carbon ?15?29Fg-1?. Specially, Me-2//LTO achieves highest capacitance of 39 F g-1 at a current density of 0.5Ag-1.?2? To increase the utilization of pores and improve the rate capability, carbon nanosheet with controllable thickness and porous structure was designed. The LIC with carbon nanosheet as positive electrode material and Li₄Ti₅O₁₂ as negative electrode material can obtain a capacitance of 55 F g-1 at 0.5 A g-1. The capacitance still retains at 88% after 3000 charge-discharge cycles at 1 A g-1. The LIC based on carbon nanosheet achieves an energy density of 77 Wh kg-1 and power density of 3409 W kg-1. By studying stepwise electrochemical behavior and diffusion kinetic of positive electrode, it is proved that the carbon nanosheet has a high utilization of surface area and fast ionic diffusion, which is ascribed to its open porous structure and short diffusion pathway.?3? To increase the capacitance of negative electrode and broaden the potential window, a metal oxide-carbon composite ?SnO2-C? with low electrode potential was introduced. The SnO₂-C composite was prepared by in-situ hydrolysis. The potential window of LIC reaches to 0.5?4V. And it achieves an energy density of 110Whkg-1 and a power density of 2960Wkg-1.The retention is 80% after 2000 charge-discharge cycles at 1 A g-1. Composite with a high content of SnO₂ trends to achieve a high initial capacitance. The Warburg impedance and ionic diffusion coefficiency were calculated by Nyquist plots of EIS. The results show that a lower SnO2 content contributes to a higher ionic diffusion rate and better cycling stability. C//SnO2?57?-C with a proper SnO2 content of 57 wt% has a high initial capacitance, improved rate capability and good cycling stability.?4? In order to further increase the capacitance of positive electrode, sulfur was impregnated into the ordered mesopore of carbon by melt-impregnation method to synthesize a C-S composite. As a positive electrode material, the C-S composite achieves a high capacitance due to the redox reaction of sulfur. The C-S electrode was also coated with polyaniline to improve the conductivity and prevent the dissolution of sulfur. The electrochemical results demonstrate that the introduction of sulfur can contribute to the capacitance of LICs, relieve the shuffle effect and improve electronic/ionic transfer rate. The LIC based on TMC-S@PANI positive electrode and SnO2-C negative electrode has a capacitance of 137 F g-1 at 0.1 A g-1 and achieves a energy density of 238 Wh kg-1. The retention of capacitance is 76% after 1500 charge-discharge cycles at increasing current densities.